Philips Lumileds Lighting Company, LLC has developed a technique for forming high efficiency LEDs, where the LEDs are formed as flip-chips and the growth substrate is removed after the flip-chip is mounted on a submount. In a flip-chip, both the n and p contacts are formed on the same side of the LED die opposite to the growth substrate side.
Prior art FIGS. 1-3 illustrate the general substrate lift-off process and the problems associated with adding an optical element in place of the removed substrate. Further details can be found in the assignee's U.S. patent Publications US 2006/0281203 A1 and 2005/0269582 A1, both incorporated herein by reference.
In FIG. 1, LED semiconductor epitaxial layers 10, including an n-layer, an active layer, and a p-layer, are grown on a growth substrate 12, such as a sapphire substrate. In the example, the layers 10 are GaN based, and the active layer emits blue light.
Metal electrodes 14 are formed that electrically contact the p-layer, and metal electrodes 16 are formed that electrically contact the n-layer. In the example, the electrodes are gold bumps that are ultrasonically welded to anode and cathode metal pads 18 and 20 on a ceramic submount 22. The submount 22 has conductive vias 24 leading to bottom metal pads 26 and 28 for bonding to a printed circuit board.
An underfill material 30 is then injected under and around the LED for structural support, to fill in air gaps, and to protect the chip from contaminants. The underfill 30 may be liquid silicone that is then cured to harden.
The substrate 12 is then removed using a laser lift-off process. The photon energy of the laser (e.g., an excimer laser) is selected to be above the band gap of the LED material and below the absorption edge of the sapphire substrate (e.g., between 3.44 eV and 6 eV). Pulses from the laser through the sapphire are converted to thermal energy within the first 100 nm of the LED material. The generated temperature is in excess of 1000° C. and dissociates the gallium and nitrogen. The resulting high gas pressure pushes the substrate away from the epitaxial layers to release the substrate from the layers, and the loose substrate is then simply removed from the LED structure. The underfill helps prevent the thin LED layers from breaking under the high pressure.
The growth substrate may instead be removed by etching, such as reactive ion etching (RIE). Other techniques may be used depending on the type of LED and substrate. In one example, the substrate is Si-based and an insulating material between the substrate and the LED layers is etched away by a wet etch technique to remove the substrate.
The exposed LED material may be further etched to remove damaged material and to thin the LED to enhance the light output. FIG. 2 shows the resulting structure.
Since the underfill 30 originally covered the sides of the substrate 12, the edges of the underfill remain after the substrate is removed to effectively form walls around the LED layers 10. Providing a precise amount of underfill to only fill under and around the LED layers without contacting the substrate is extremely difficult and so the structure of FIG. 2 is typical. The underfill 30 typically extends further laterally than shown in the figures in actual devices.
As shown in FIG. 3, a transparent adhesive material 32 (e.g., silicone) is deposited on the exposed LED surface layer. A preformed phosphor plate 34 is intended to be precisely positioned over the LED and glued to the top surface of the LED. Any misalignment of the plate 34 causes the plate 34 to not seat properly on the LED due to the raised underfill walls around the LED. The phosphor plate 34 may be formed of a YAG phosphor either sintered or in a transparent binder. The YAG phosphor emits yellow-green light when energized by the blue LED. The yellow-green light combined with the blue light leaking through the plate 34 produces white light. As a result of the plate 34 not seating properly, the light emission properties of the LED will not be optimal and the plate 34 may easily delaminate from the LED.
What is needed is an improved technique that avoids the above-mentioned alignment problems when affixing a phosphor plate or any other optical element to the surface of the LED in place of the removed substrate.